An antilocking brake fluid pressure control unit for a vehicle according to the present invention is located between a master cylinder which is directly coupled with a brake pedal and a piston which is directly coupled with a wheel brake in a braking system for an automobile, for controlling the braking system in an antilocking manner. When a wheel speed detector or a body speed detector detects an excessive slip caused by overbraking or an indication thereof, the antilocking brake fluid pressure control unit drives electromagnetic valves to reduce or control the brake fluid pressure, thereby suppressing or controlling the braking force to an optimum level.
In connection with such an antilocking brake fluid pressure control unit, there has already been developed a rotary flow type unit, which is adapted to feed back a brake working fluid discharged from a wheel cylinder of a wheel brake, by a pump provided on an intermediate portion of a circulation passage for reusing the working fluid. In such a rotary flow type antilocking brake fluid pressure control unit, however, a working noise of the pump, vibration of a piping system caused by pulsation of the working fluid in the circulation passage and a following working noise, or vibration of the pedal inevitably leads to a large noise.
In order to solve such a problem, British Patent No. 1,590,003, for example, discloses a system which comprises a damping chamber provided between an outlet port of a pump and a feedback point for storing a constant volume of a working fluid and an orifice provided in the vicinity of the outlet of this damping chamber for damping any pulsation of the pump through the compressibility of the working fluid.
The antilocking brake fluid pressure control unit according to the aforementioned prior art is applied to a braking system shown in FIG. 4, for example. FIG. 4 shows only an antilocking brake fluid pressure control unit interposed between a master cylinder 2 which is directly coupled with a brake pedal 1 and a wheel cylinder 4 of a wheel brake 3 for performing an antilocking operation. In practice, however, the master cylinder 2 is connected also with other antilocking brake fluid pressure control units which are interposed between the master cylinder and the wheel cylinders of the other three wheels. In the antilocking brake fluid pressure control unit shown in FIG. 4, a two port, two-position electromagnetic valve 6 for controlling the introduction of a working fluid is provided in an intermediate portion of a main fluid passage 5 connecting a master cylinder 2 with a wheel cylinder 4 of a wheel brake 3. The electromagnetic valve 6 is closed only when the magnetic valve coil is energized. The valve 6 is maintained in an open state when the valve coil is deenergized. A circulation passage 7 branches from the main fluid passage 5, so that another two-port, two position electromagnetic valve 8 is provided on an intermediate portion thereof for controlling the discharge of working fluid. This electromagnetic valve 8 is opened only when its coil is energized. The valve 8 is maintained in a closed state when its coil is deenergized. Another circulation passage 9 is provided between a downstream end of the circulation passage 7 and a feedback point 15 in an upstream portion of the main fluid passage 5, while a reservoir 10 is connected to a coupling portion 9 between the circulation passages 7 and 9 for storing the working fluid, which is discharged from the wheel cylinder 4 to flow through the circulation passage 7.
The circulation passage 9 is provided with a pump 12 which is driven by a motor 11, while a damping chamber 13 for storing a constant volume of the working fluid and an orifice 14 are provided downstream of the pump.
The operation of the conventional antilocking brake fluid pressure control unit having the aforementioned structure is now described. When leg power is applied to the brake pedal 1, the master cylinder 2 converts this power to the fluid pressure of the working fluid in response to the amount of actuation. If the unit is in a non-antilocking state, the electromagnetic valves 6 and 8, the coils of which are deenergized, are in open and closed states respectively. When leg power is applied to the brake pedal 1 in this state, the fluid pressure which is converted by the master cylinder 2 in response to the amount of actuation is supplied through the main fluid passage 5 to the wheel cylinder 4 of the wheel brake 3, to drive the wheel brake 3.
When a wheel speed detector or a body speed detector detects an overbraking operation, this unit receives an electric signal to enter an antilocking state, thereby supplying power to both electromagnetic valves 6 and 8.
Therefore, the electromagnetic valves 6 and 8 enter open and closed states respectively to cut off the fluid pressure from the master cylinder 2 and open the circulation 7, thereby attaining a decompressed state in the wheel cylinder 4 of the wheel brake 3. Thus, a brake locking is prevented that could otherwise be caused by an overbraking operation.
In order to re-pressurize the brake fluid pressure, the power supply is stopped to open and close the electromagnetic valves 6 and 8 respectively. In order to maintain the brake fluid pressure at a constant level, further, power is supplied only to the electromagnetic valve 6 for closing the same, while no power is supplied to the electromagnetic valve 8 for maintaining the same in the closed state. Consequently, portions of the main fluid passage 5 and the circulation passage 7 are sealed around the wheel brake 3, to maintain the fluid pressure at a constant level.
The working fluid discharged through the circulation passage 7 and the electromagnetic valve 8 during the antilocking operation is temporarily stored in the reservoir 10. The pump 12 appropriately sucks the working fluid stored in the reservoir 10, and returns the working fluid to the feedback point 15 through the circulation passage 9, thereby reusing the working fluid. At this time, the damping chamber 13 and the orifice 14 provided immediately at the back of the pump 12 absorb a pressure fluctuation which may be caused by pulsation of the pump 12, to relieve any influence on the piping of the circulation passage 9.
However, the rotary flow type antilocking brake fluid pressure control unit of the aforementioned structure has the following problems:
When assembling a vehicle, a brake piping system is temporarily evacuated and then charged with a working fluid under pressure through so-called vacuum charging, so that air is bled from the piping system. If the working fluid remains in the brake piping system, a desired degree of vacuum cannot be attained since the remaining working fluid is vaporized upon exhaust. Also in the antilocking brake fluid pressure control unit shown in FIG. 4, it is necessary to discharge the working fluid after charging the same in the manufacturing step, for performance investigation. In that case, it is difficult to discharge the working fluid from the damping chamber 13, due to provision of the orifice 14. This problem is particularly remarkable when the damping chamber 13 is provided in the vicinity of the orifice 14. In order to completely discharge the working fluid from the damping chamber 13, a fluid discharge port may be provided to be sealed with a tap after discharging the working fluid. In this case, however, costs are increased.
Further, when servicing a vehicle with a brake system as described above, it takes a long time to bleed air only by forcefeed power caused by leg power which is applied to the brake pedal due to an inferior air bleeding ability of the known system. The term "servicing" herein used means that a dealer repairs parts of an automobile requiring such repairs in accordance with a service manual.
In order to solve the aforementioned problems caused in connection with the vacuum charging of the working fluid in the brake piping system, a technique proposed in Japanese Utility Model Laying-Open No. 1-103467 (1989), for example, may be applied to provide a check valve in a portion of the circulation passage 9 between the pump 13 and the feedback point 15 to allow only a flow of the brake fluid from the pump 13 toward the feedback point 15. However, such use of a high-priced check valve also leads to an increase in the costs.